1. Field of the Invention
The invention relates to a method of processing mailpieces, which method, during a first mailpiece sorting pass, comprises forming a digital image of the surface of each mailpiece that bears information blocks, in deriving a digital signature as a kind of graphical fingerprint from each image associated with a mailpiece, which signature is a logical identifier for the mailpiece, and in recording the signature associated with the mailpiece in a memory in correspondence with sorting information, and, during a second mailpiece sorting pass, comprises forming a digital image of said surface of each mailpiece again in order to derive a current digital signature associated with the mailpiece, and in searching through the signatures recorded in the first sorting pass for a match with said current signature in order to retrieve the sorting information by association.
2. Discussion of the Background Art
Such a method is known from French Patent Document FR-2 841 673. With that method, it is no longer necessary to affix Id tags constituted by identification codes or time stamps on the surfaces of the mailpieces. The mailpieces are identified by means of respective “virtual” codes that offer the advantage of obviating the need to use bar code printers and thus of significantly reducing the operating and maintenance costs for postal sorting equipment.
In that known method, each digital signature comprises a first component or “image component” representative of physical characteristics of the digital image of the mailpiece corresponding to a second component or “postal component” indicating at least one spatial position of the information blocks present in the image of the mailpiece. In particular, the image component is formed by “global” attributes that are representative of overall physical characteristics taken from the entire set of picture elements (pixels) of the digital image of the mailpiece. The image component is also formed of second attributes or “local” attributes that are representative of local physical characteristics taken from distinct portions of a grid applied over the image of the mailpiece (or of a plurality of different grids).
In practice, when searching for a match between a current signature and a candidate signature recorded in a signature database for the purpose of retrieving sorting information, firstly the respective image components of the signatures are compared and then the respective postal components of the signatures are compared.
When batches of mailpieces coming from bulk senders or “bulk mailers” are to be sorted by using such virtual identification codes or signatures, the problem arises that, overall, the image components (global and local attributes) of the signatures associated with the mailpieces from the same sender cannot serve to discriminate between the signatures. The mailpieces coming from the same bulk sender are generally identical graphically: same type of envelope, same logo printed on the envelope, same sender address block, same position of the recipient address block, etc. Therefore, in practice, it is possible to distinguish between two signatures of mailpieces from the same bulk sender only by the contents of their respective recipient address blocks.
However, in forming signatures, it is not impossible that situations might arise in which the data processing system identifies a sender address block instead of a recipient address block. As a result, when searching through the candidate signatures associated with mailpieces from the same sender, it is possible that a match might be found between two signatures that have the same image component and that, in addition, have postal components that are identical due to the fact that they both erroneously identify sender address blocks instead of recipient address blocks.
In such a situation, an increase in signature matching errors can be observed when searching for matches. Such matching errors naturally give rise to errors in directing the mailpieces to the sorting outlets, and thus to additional costs for the mail handling performed with a view to delivering the mail.